Method of manufacturing a ferritic-martensitic, oxide dispersion strengthened alloy

ABSTRACT

This invention relates to a method of manufacturing an improved ferritic or martensitic alloy based on iron and chromium strengthened by a dispersion of oxides, commonly called an Oxide Dispersion Strengthened or ODS alloy, and, more particularly to a method of manufacturing a ferritic or martensitic ODS alloy with large grains based on iron and chromium which has a single phase ferritic or martensitic matrix having an isotropic microstructure and a grain size that is sufficient to guarantee mechanical strength compatible with a use of this alloy at high temperature and/or under neutron irradiation. According to the invention, the method comprises slow cooling of an austenite at a cooling rate less than or equal to the critical cooling rate for transformation of this austenite into ferrite.

TECHNOLOGICAL FIELD OF THE INVENTION

This invention relates to a method of manufacturing an improved ferriticor martensitic alloy, including chromium and which is strengthened by adispersion of oxides, commonly called an ODS (Oxide DispersionStrengthened) alloy, and more particularly, to a method of manufacturinga ferritic or martensitic ODS alloy with large grains based on iron andchromium which has single phase ferritic or martensitic matrix having anisotropic microstructure and a grain size that is sufficient toguarantee mechanical strength compatible with a use of this alloy athigh temperature and/or under neutron irradiation.

ODS alloys are made up of a metal matrix having a body-centered cubiccrystal structure. This structure is strengthened by a dispersion ofoxides of the type Y₂O₃, TiO₂ etc. which gives it excellent mechanicaland chemical properties at medium and high temperatures.

The resistance to oxidation of these alloys is due particularly to thepresence of chromium. This resistance is only effective when theconcentration of chromium is greater than 8% by weight in the alloy.However, when this concentration is greater than 12% by weight, thealloy becomes brittle.

Furthermore, thanks to their crystal structure, these alloys have goodresistance to swelling and to creep under neutron irradiation.

These alloys can be used, for example, as structural materials forcomponents in the core of a nuclear power station since these componentsmust have a high mechanical strength at high temperature, for examplefrom 400 to 700° C., must be resistant to neutron radiation, must becompatible with use in a sodium environment and resistant to oxidationetc.

In a general way, these alloys are also useful in the manufacture ofcomponents subject to high mechanical and thermal stresses such ascomponents of thermal power stations, components used in the glass, gasor aeronautical industries etc.

PRIOR ART

Many types of ODS alloys that include chromium have already beendeveloped in the prior art. These have chromium concentrations between13 and 20%, variable contents of Mo, W, Al and Ti, and a small quantityof carbon, generally less than 0.02% by weight (200 ppm). In this typeof alloy, the matrix is totally ferritic whatever the heat treatmenttemperature.

Hence, American U.S. Pat. No. 4,075,010 describes an alloy having acomposition Fe-14 Cr-1 Ti-0.3 Mo-0.25 Y₂O₃.

This alloy displays a very good compromise between strength andductility in a direction parallel to the axis of forming of the alloy.However the grains that make it up are elongated in the direction offorming which leads to a high degree of anisotropy in its mechanicalproperties. This anisotropy leads to too low a mechanical strength alongdirections perpendicular to the direction of forming. Such an alloy cantherefore not be used, for example to make cladding tubes for nuclearreactors, since the radial direction is the main direction of mechanicalstress in these tubes in a reactor. In addition, this alloy contains ahigh level of chromium which causes it to become brittle under neutronradiation through the precipitation of phases rich in this element.

This type of alloy generally produced by mechanical alloying from itsconstituents starting with elemental or pre-alloyed powders. In thistype of alloy, mechanical alloying is a method that allows one tointroduce into the metal matrix, the fine and homogeneous distributionof oxides that confers a very high hot strength on the alloy. Thepowders thus provided are compacted and drawn at high temperature andpressure.

This method of production however produces an alloy in which the meangrain size is generally too small, that is to say less than 1 μm andwhich has an anisotropic microstructure when the initial chemicalcomposition of the matrix means it has a ferritic structure. Under theseconditions, too small a grain size causes a reduction in the mechanicalstrength of the alloy, particularly at high temperatures greater than500° C. Furthermore, the anisotropy of the grain size leads toanisotropy in the mechanical properties of the alloy.

In particular, the initial ferritic structure is inevitable in examplesthat contain more than 12% of chromium.

In order to avoid these problems of anisotropy, a man skilled in the arthas been drawn into using a martensitic material less rich in Cr, but inthis case, control of the mean grain size has proved to be impossible.In effect, in this type of material, no variation in grain sizewhatsoever has been observed after traditional heat treatment even attemperatures as high as 1250° C.

Patent application GB-A-2 219 004 describes an ODS alloy with a temperedmartensitic matrix having a chromium concentration of from 8 to 12% byweight and concentrations of (Mo+W) and of carbon respectively between0.1 and 4% and 0.05 and 0.25% by weight. In addition, the alloydescribed is strengthened by a dispersion of Y₂O₃ and TiO₂ oxideparticles at a concentration of from 0.1 to 1% by weight. Theapplication examples described in this document include a chromiumconcentration greater than 10% by weight and a concentration of Mo and Wbetween 2 and 4% by weight. The preparation method of the alloydescribed comprises mechanical alloying of the alloy in an attritor,compaction of the alloy under vacuum and hot drawing at a temperatureranging from 900 to 1200° C. This procedure is followed by anormalization treatment at a temperature ranging from 950 to 1200° C.and a tempering at a temperature ranging from 750 to 820° C.

However the method described does not allow one to control the grainsize of the alloy.

The prior art methods therefore all have one or more of the followingdisadvantages

they do not allow one to obtain an isotropic microstructure of theformed alloy,

they do not allow one to specify and control the grain size of the alloy

they lead to a grain size that remains too small.

As a consequence, the prior art alloys all have one or more of thefollowing disadvantages:

insufficient mechanical strength at high temperature due to theanisotropy of its microstructure,

embrittlement at high temperature and/or under neutron irradiationthrough precipitation of embrittling phases in the alloy dueparticularly to an excess of chromium, and

a mechanical strength that is not always compatible with use at hightemperature and/or under neutron irradiation due particularly to havingno control over the grain size of the alloy and to the grain size beingtoo small.

DESCRIPTION OF THE INVENTION

The precise purpose of this invention is to provide a method ofmanufacturing a ferritic ODS alloy that includes chromium, and a methodof manufacturing a martensitic ODS alloy that includes chromium, whichdoes not have the disadvantages mentioned above and which in particular,allows one to specify and control the grain size of the alloy producedby exercising control over successive phase transformations.

The method, according to the invention, of manufacturing an alloy with aferritic ODS structure that includes chromium, comprises preparation ofa martensitic ODS blank that includes chromium and a step consisting ofsubjecting the martensitic ODS blank to at least one thermal cyclecomprising an austenitization of the martensitic ODS blank at atemperature greater or equal to the AC3 point of this alloy in such away as to obtain an austenite, followed by cooling of this austenite ata slow cooling rate that is less than or equal to the critical coolingrate for transformation of this austenite into ferrite in such a way asto obtain an alloy with a ferritic structure, said slow critical coolingrate being determined from a phase transformation diagram for thisaustenite under continuous cooling.

The alloy manufactured by the method of the invention is notablyimproved since it has a single phase ferritic matrix having an isotropicmicrostructure and a grain size that is sufficient to guaranteemechanical strength compatible with use of this alloy at hightemperature and/or under neutron irradiation. Furthermore, it hassufficient ductility to be subjected to forming even at ambienttemperature.

According to the invention the martensitic ODS blank that includeschromium can be prepared by any method that allows one to obtain a blankin which the oxides are dispersed in the metal matrix in a fine andhomogeneous manner.

In an advantageous way, the blank can be prepared from a pre-alloyedpowder obtained by mechanical alloying. In effect the mechanicalalloying makes possible the dispersion of oxides such as Y₂O₃ requiredfor the preparation of an ODS alloy.

The mechanical alloying can be carried out in an attritor, under aneutral atmosphere, for example argon, starting with a powder, obtained,for example, by atomization of an ingot under argon, that has acomposition which corresponds to that of the manufactured blank and byadding oxides such as Y₂O₃. The powder can also be obtained by mixingpure or pre-alloyed powders available on the market.

According to the invention, the martensitic ODS blank can be prepared bya traditional technique of consolidating a pre-alloyed powder forexample by drawing and hot forming.

According to the invention, the martensitic ODS blank that includeschromium can also include one or more elements chosen from the groupcomprising Mo, W, Ni, Mn, Si, C, O, N, Y, Ti, Ta, V, Nb, Zr. Forexample, it can include one or more of the oxides currently used formanufacturing alloys strengthened by a dispersion of oxides, for exampleY₂O₃, TiO₂, MgO, Al₂O₃, MgAl₂O₄, HfO₂, ThO₂ and ZrO₂. The function ofeach of these elements and oxides in the martensitic ODS blank is knownto a man skilled in the art. As a consequence, it will not be describedhere.

According to the invention, the martensitic ODS blank that includeschromium can, for example, include from about 7 to about 12% by weightof chromium equivalent in the alloy, for example, to about 8 to about11% by weight, that is to say a quantity of alphagenic elementsequivalent to a quantity of chromium of about 7 to about 12% by weight,for example, from about 8 to about 11% by weight. The alphagenicelements are notably elements that allow one to reduce the area of thedomain in which the austenite exists. The method according to theinvention can therefore be applied advantageously to a type 9 Cr alloy.

According to the invention, the chromium can be at a concentration offrom about 7 to about 12% by weight, for example from about 8 to about12% by weight, Mo can be at a concentration of from about 0.3 to about1.5% by weight, W can be at a concentration of from about 0.5 to about3% by weight, Ni can be at a concentration ranging up to about 1% byweight, Mn can be at a concentration ranging up to about 1% by weight,Si can be at a concentration ranging up to about 1% by weight, C can beat a concentration of from about 0.02 to about 0.2% by weight, O can beat a concentration of from about 0.02 to about 0.3% by weight, N can beat a concentration ranging up to about 0.15% by weight, Y can be at aconcentration ranging up to about 1% by weight and Ti can be at aconcentration ranging up to about 1% by weight in the alloy, theremainder being iron.

According to the invention, this alloy can also include Ta and Nb, eachat a concentration ranging up to about 0.2% by weight, V at aconcentration ranging up to about 0.4% by weight, and Zr at aconcentration ranging up to about 0.4% by weight.

The, at least one, thermal cycle according to the invention allows oneto induce, in the martensitic ODS blank, a transformation of themartensite into austenite, then a slow transformation of the austeniteinto ferrite with a stable grain growth at low temperature.

According to the invention the, at least one, thermal cycle comprises anaustenitization of the martensitic ODS blank at a temperature greaterthan or equal to the AC3 point for this alloy in such a way as to obtainan austenite. The AC3 point of such an alloy corresponds to thetemperature at which the ferrite completely transforms itself intoaustenite in the course of the heating. When the alloy is an alloy suchas those described above, this austenitization can be carried out at atemperature of from about 950 to about 1150° C., for example, at atemperature of from about 1000 to about 1100° C., for example at atemperature of about 1000° C. and for a period of from about 15 to about120 minutes, for example, from about 30 to 90 minutes, for example aperiod of about 30 minutes, a period of less than 15 minutes often beinginsufficient to obtain an austenitic structure, and a period greaterthan 120 minutes not being necessary since the austenitic structure hasoften been obtained earlier.

According to this invention, this austenitization is followed by acooling of the austenite obtained, at a slow cooling rate less than orequal to the critical rate of cooling for transformation of thisaustenite into ferrite, said slow cooling rate being determined from aphase transformation diagram for this austenite under continuouscooling. This phase transformation diagram, or TRC, can be obtained in ausual way.

The rate of cooling of the austenite is said to be “slow” in thisdescription so as to differentiate it from the “fast” rate of coolingfor a martensitic transformation described below in the method ofmanufacturing a martensitic ODS alloy according to the invention.

This slow cooling induces a transformation of the austenitic phase whichis a high temperature phase into a ferritic phase which is morepropitious than martensite for grain growth.

The slow rate of cooling can, for example, be less than or equal to 280°C. per hour, for example, for an alloy composition such as thosepreviously described. For example, for at least one thermal cycle, itcan be less than about 250° C. per hour, for example, less than or equalto about 100° C. per hour, for example, less than or equal to about 20°C. per hour. It should further be noted that the cooling rate depends,not only on the composition of the alloy manufactured, but also on theaustenitization temperature of this alloy. It will be easily understoodthat in accordance with the constraints of industrial manufacture and inaccordance with the composition of the alloy, a man skilled in the artcan adapt the austenitization temperature and the slow rate of coolingof the method of the invention.

The inventors have observed that when the cooling rate is less than thecritical rate of phase transformation of the austenite, there is anincrease in the grain size in the alloy. They have also surprisinglyobserved that the lower the rate of cooling, the greater the graingrowth in the alloy.

Very good grain growth has, for example, been observed by the inventorswith a cooling rate of from about 5 to about 20° C. per hour. Forexample, for an alloy composition such as those previously described, aslow cooling rate less than 100° C. per hour gives an increase in thegrain size of the manufactured ferritic alloy such that the mean grainsize reaches 3 to 8 μm in this alloy.

This cooling can be controlled, for example, as far as 650° C. for thecompositions previously mentioned, that is to say as far as thetemperature at which the phase transformation is finished.

In this example, below 650° C., fast cooling can be applied.

After a first thermal cycle according to the invention, the martensiticODS blank is transformed into an alloy with a ferritic ODS structurewith a grain size greater than the old grain size of the austenite.

According to the invention, the thermal cycle can be repeated severaltimes, with identical or different slow rates of cooling, which allowsone to obtain additional growth in the grain size of the alloy with aferritic ODS structure formed during a first cycle. It can be repeateduntil this growth ceases, that is to say until an optimization of thegrain size of the ODS alloy is achieved. According to the invention, thethermal cycle can be, for example, repeated two, three or four timeswith an alloy composition such as those previously mentioned,optimization being obtained at the end of four cycles in this example.

For example, for an alloy composition such as that previously described,and for a single cycle comprising a cooling at a slow cooling rate ofabout 6° C. per hour, the ferritic alloy manufactured according to theinvention can have a mean grain size approximately equal to 8 μm. Forexample, for one and the same alloy composition and for such a cyclerepeated four times, the ferritic alloy manufactured according to theinvention can have a mean grain size up to about 10 μm and even more.

Therefore the method of the invention allows one to obtain an optimizedferritic structure with large grains.

According to a first variant of the method of the invention, the methodof manufacture of an alloy with a ferritic ODS structure that includeschromium can include at least two thermal cycles according to theinvention, said thermal cycles being separated by at least one formingtreatment of the alloy obtained with a ferritic ODS structure.

On the one hand, and as previously described, the thermal cycleaccording to the invention permits growth in the grain size of thealloy. On the other hand, this, at least one thermal cycle, allows oneto obtain a ferritic ODS structure which permits, particularly throughits ductility, the feasibility of the forming treatment according to thevariant of the method of the invention. In effect, this, at least onethermal cycle, for example with an alloy having a composition such asthose mentioned above, allows one to obtain a hardness less than orequal to 240.

The forming treatment according to the invention can comprise a formingof the ferritic ODS alloy and possibly a thermal stress relievingtreatment of this alloy. The forming of the ferritic ODS alloy can be,for example, drawing, hammering, spinning, rolling and in a general way,any forming process that allows one to form sheet, tubes or othercomponents from this alloy at a temperature ranging, for example up to800° C. This forming can, for example be carried out by drawing orrolling in order to form cladding tubes for nuclear fuel. According tothe invention, the manufactured alloy with a ferritic ODS structure issufficiently ductile to be cold formed.

Hence thanks to the method of this invention, the forming can, forexample, be carried out at ambient temperature.

According to the invention, the forming treatment can includeadditionally a thermal stress relieving treatment of the formed alloy,at a temperature less than AC1.

This thermal stress relieving treatment can be, for example, a classicsoftening treatment of an alloy. In particular, it allows a release ofthe residual stresses after forming of the alloy without any change inthe structure of it.

The AC1 temperature is the temperature at which austenite begins to beformed during heating. For example, in the case of an alloy compositionsuch as those previously described AC1 is equal to 775° C. Also, in thisexample, the thermal softening treatment can be carried out at atemperature lower than about 775° C. for example, at a temperatureranging from about 720 to about 750° C.

According to the invention, the thermal stress relieving treatment canbe carried out for a period of from about 15 to about 120 minutes, forexample, for a period of about 60 minutes.

This intermediate step of forming the ferritic ODS alloy thereforeallows one to obtain an alloy, for example formed into a tube or intosheet, having a ferritic structure with a grain size greater than orequal to about 1 μm, for example, about 3 μm, for an alloy compositionsuch as those previously described.

According to the invention, this alloy with a ferritic ODS structurehaving been subjected to at least one forming process can then besubjected to at least one thermal cycle according to the invention, inorder to optimize the grain size of its structure, for example, up toabout 10 μm in the previous example.

Advantageously, according to the method of the invention, the, at leastone forming treatment of the ferritic ODS alloy obtained can be athermal cycle comprising a slow rate of cooling, for example, of fromabout 50 to about 250° C. per hour and the, at least one thermal cyclewhich follows the forming treatment of the alloy can comprise an evenslower rate of cooling, for example of from about 20 to about 5° C. perhour. Hence the, at least one cycle that precedes the forming of thealloy allows one to rapidly form a ductile ferritic alloy and the, atleast one thermal cycle that follows the forming permits optimization ofthe grain size of the alloy.

According to an embodiment of this first variant of the method of theinvention, a martensitic ODS blank having a composition such as thosementioned previously can be subjected, for example, to a first thermalcycle according to the invention with a slow rate of cooling of about100° C. per hour in order to obtain a ductile alloy with a ferritic ODSstructure having a mean grain size approximately equal to 3 μm. Theductile alloy can then be formed by one or more forming processescomprising, for example, a cold forming process and a softeningtreatment typically for one hour at 720-750° C. The formed alloy canthen be subjected to one or more thermal cycles with a slow cooling rateof about 10° C. per hour in order to optimize the grain size of thisalloy, for example, with four thermal cycles in this example.

This embodiment example allows one, for example, to manufacture a formedalloy having an optimized ferritic structure with large grains, having asize of about 10 μm.

The invention also relates to a method of manufacturing a martensiticODS alloy that includes chromium, said method comprising a method ofmanufacturing an alloy with a ferritic ODS structure according to theinvention, followed by a martensitic transformation step and a temperingof the manufactured martensitic ODS alloy, said martensitictransformation step comprising an austenitization of said alloy with aferritic ODS structure at a temperature greater than or equal to the AC3point of this alloy in such a way as to obtain an austenite, followed bycooling at a fast rate of cooling greater than or equal to the criticalcooling rate for transformation of austenite into martensite, said fastrate of cooling being determined from a phase transformation diagram forthis alloy under continuous cooling.

This method according to the invention allows one to obtain, from analloy having a ferritic ODS structure, such as that previouslydescribed, an alloy with a martensitic ODS structure and with largegrains. The conversion to a ferritic ODS structure according to theinvention permits forming of the alloy even at ambient temperature.

According to the invention, the austenitization of the alloy with aferritic ODS structure and with large grains at a temperature greaterthan or equal to the AC3 point can be such as that previously described.

According to the invention, the critical cooling rate for transformationof austenite into martensite can be determined from a TRC diagram suchas that previously described. This rate is referred to as being “fast”in this description for the reason previously given. This fast rate canbe, for example, greater than or equal to about 700° C. per hour for acomposition such as those previously described.

According to the invention, the tempering can be a usual temperingtreatment, for example, tempering carried out at a temperature of about750° C. for about 1 hour. It permits stress relief of the structure.

This method allows one to manufacture a martensitic ODS alloy having anisotropic microstructure and a mean grain size that is sufficient toguarantee a mechanical strength compatible with a use of this alloy athigh temperature, for example, greater than 400° C., for example between400 and 700° C. and/or under neutron irradiation. This mean grain sizeis equivalent to that obtained in the ferritic ODS structure by themethod of this invention.

Therefore, this method allows one, for example, to manufacture an alloyto be used for components of a nuclear power station subjected to hightemperatures and/or neutron radiation, for example, cladding tubes fornuclear fuel. However it is not limited to the manufacture of thesecomponents, it also allows one to manufacture in a general way, anycomponent subjected in its use to high mechanical and thermal stresses,for example, turbine blades in the aeronautical industry, components ofa thermal power station, items used in the glass and gas industries etc.

The invention also relates to an alloy with a ferritic ODS structurethat includes chromium, or a martensitic ODS structure that includeschromium, that can be manufactured by the method of the invention havinga mean grain size greater than 1 μm, to such an alloy having a meangrain size greater than 5 μm, and to an alloy having a mean grain sizeranging up to about 10 μm or more.

According to the invention, this alloy with a ferritic ODS structure ora martensitic ODS structure can be, for example, chosen from a groupcomprising an 9 Cr alloy, a 9 Cr—Mo alloy, a 9 Cr—W alloy or a 9 Cr—Mo—Walloy. Alloys of the 9 Cr—W type are called “low activity alloys” sincethey comprise elements with a short radioactive decay time. Thesealloys, conforming to this invention are therefore of interestparticularly for the manufacture of components used in nuclear powerstations.

According to the invention, these alloys can comprise, in addition, forexample, at least one element chosen from the group comprising Cr, Mo,W, Ni, Mn, Si, C, O, N, Y and Ti, Ta, V, Nb and Zr. The concentration ofeach of these elements in the alloy can be that described previously inthe method according to the invention.

The alloy according to the invention can therefore, for example, be ofuse in the manufacture of nuclear fuel cladding, and in a general waycomponents such as those previously described.

The alloys of this invention are notably very strong mechanically andhighly resistant chemically at high temperature and/or under neutronirradiation. In addition they have an isotropic structure with largegrains and a reduced quantity of chromium.

Other advantages and characteristics of this invention will becomeapparent on reading the description that follows, given, it isunderstood for illustrative purposes and being non-limitative, whichmakes reference to the appended Figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a graph illustrating thermal cycles using different coolingrates to induce the transformation from an austenite to a ferrite,

FIG. 2 is a graph illustrating the change in mean grain size as afunction of the cooling rate

FIG. 3 is a graph illustrating several applications of a thermal cyclewith a cooling rate of 6° C. per hour,

FIG. 4 is a graph illustrating the change in mean grain size andhardness as a function of the number of thermal cycles shown in FIG. 3.

EXAMPLE 1

Preparation of Ferritic-martensitic Allows and Examples of Fe-9 Cr-1 Moalloys

The alloys manufactured in this example, according to the method of theinvention, are alloys of the 9 Cr-1 Mo type, based on iron, named EM10in what follows, and strengthened by particles of yttrium oxide.

Each of these alloys is manufactured by a mechanical alloying method. Aningot having the composition of the alloy desired is atomized underargon in such a way as to obtain a 9 Cr-1 Mo pre-alloyed powder. Thispre-alloyed powder is ground and mixed with yttrium oxide (Y₂O₃) inpowder form, in an attritor under an atmosphere of argon. Then theresulting powder is compacted by hot drawing at a temperature of 1100°C. at a drawing ratio of between 15 and 30 in order to obtain amartensitic blank that includes chromium strengthened by the dispersionof oxide particles.

Several alloys are manufactured. The chemical composition of thesealloys is shown in Table 1 below. When the alloy contains dispersedoxides such as Y₂O₃ these are added during the mechanical alloying whenthe pre-alloyed powder is ground in the attritor. These alloys are namedEM10+Y₂O₃−ODS and EM10+Y₂O₃+Ti−ODS below. The alloy that does notcontain any oxide is named atomized EM10 below.

Table 1 below shows both the composition of a conventional EM10 alloy,that is to say an alloy obtained by a method other than mechanicalalloying, for example, by a melting process, and the composition of anFe—13Cr alloy of the type with an oxide dispersion, availablecommercially and named below alloy ODS-MA957 (registered trade mark)from the company INCO ALLOYS (USA).

TABLE 1 Chemical composition of alloys according to the invention, aconventional alloy and ODS-MA957 alloy Materials Cr Mo Ni Mn Si C O N YTi Conventional EM10 9.0 1.1 0.6 0.6 0.4 0.11 <0.004 0.025 Atomized EM108.50 1.00 0.53 0.47 0.37 0.088 0.019 0.015 — — EM10 + Y₂O₃ − ODS 8.401.14 0.53 0.49 0.37 0.103 0.129 0.025 0.17 — EM10 + Y₂O₃ + TiO₂ − ODS8.38 1.13 0.52 0.50 0.37 0.122 0.133 0.024 0.15 0.23 ODS-MA957 12.6 0.3— — — 0.012 0.18 — 0.18 0.88

The temperatures AC1 and AC3 of the alloy ODS−EM10+Y₂O₃ the compositionof which is given in Table 1 are respectively within the ranges 775-800°C. and 815-840° C.

A phase transformation diagram under continuous cooling (TRC) obtainedafter austenitization of this EM10+Y₂O₃−ODS alloy at 1000° C. for 30minutes has allowed one to determined the critical cooling rates,designated below Vr(m), to obtain a totally martensitic product, greaterthan or equal to 700° C. per hour, and the critical cooling rates,designated below Vr(α), for a total transformation of the austenite intoferrite, or (α) ferrite, less than or equal to 280° C. per hour.

Hardness values, characteristic of this EM10+Y₂O₃−ODS alloy of ferriticor martensitic structure, obtained according to the method of theinvention have been measured. For the alloy with a ferritic structure,the hardness, designated below HV(α) is less than or equal to 244, andfor the alloy with a martensitic structure, the hardness, designatedbelow as HV(m) is greater than or equal to 460.

EXAMPLE 2

Effect of the Cooling Rate According to the Invention on the Grain Sizeof the EM1030 Y₂O₃−ODS Alloy with a Ferritic Structure

In this example, the alloy manufactured is an alloy with a ferriticstructure.

A series of tests are carried out starting with a martensiticEM10+Y₂O₃−ODS blank having the composition described in Table 1, inorder to measure the effect of cooling rate on the grain size of themanufactured alloy.

Four samples of the EM10+Y₂O₃−ODS alloy were subjected to a firstthermal cycle according to the invention, comprising an austenitizationat a temperature of 1000° C. for 30 minutes followed by cooling to 650°C., at a rate that was different for each sample, and fast cooling at acommon rate of 3° C. per second from 650° C.

Table 2 below shows the cooling rates for each sample (designated E₁,E₂, E₃ and E₄) as far as 650° C.

TABLE 2 Thermal cycle using different cooling rates afteraustenitization Cooling rate to 650° C. Sample (in ° C./h) E₁ 100 E₂ 20E₃ 10 E₄ 6

FIG. 1 is a graph illustrating the thermal cycles of E₂, E₃ and E₄carried out in this example.

In this Figure, the line designated A-B-C illustrates theaustenitization of the martensitic blank, obtained by mechanicalalloying EM10+Y₂O₃−ODS, by heating to 1000° C. (line A-B), then holdingthe alloy at this temperature for 30 minutes (line B-C). This lattertemperature corresponds to a temperature greater than the AC3 point ofthe alloy.

The curves E₂, E₃ and E₄ represent respectively the cooling rates ofsamples E₂, E₃ and E₄, which are different as far as 650° C. and thenidentical from this temperature.

The grain size of the samples has been measured by an image analysistechnique, as a function of the cooling rate which characterizes eachsample.

Table 3 below gathers together the results of these measurements.

TABLE 3 Effect of the cooling rate according to the invention on thegrain size at the time of transformation of the austenite into ODSferrite Cooling rate Vr in ° C./h 100 20 12 6 Grain Size 3 4.2 4.9 8 α(μm)

FIG. 2 is a graph illustrating the effect of cooling rate according tothe invention on the grain size of the alloy manufactured, inparticular, the curve I is a graphic representation of the values givenin Table 3 above.

The results show that the lower the rate of cooling the greater theincrease in the grain size of the alloy.

These results should be compared with a grain size of the order of 1 μmor less obtained after a normal treatment of the ODS−EM10+Y₂O₃ alloy,whatever the temperature used within a range of 1000 to 1250° C.,followed by fast cooling.

EXAMPLE 3

Repetition of a Thermal Cycle According to the Invention on an Alloywith a Ferritic ODS Structure

This example uses an ODS−EM10+Y₂O₃ alloy such as that described in Table1 above.

A thermal cycle with slow cooling is applied in a repeated way on thisalloy in such a way as to measure the effect of this repetition on thegrain size of the alloy.

FIG. 3 illustrates diagrammatically this Example 3, reference number 1indicating a slow thermal cycle. This thermal cycle comprises anaustenitization which consists of heating the alloy to a temperature of1000° C., indicated by reference number 2, and of holding the alloy atthis temperature for 30 minutes, indicated by reference number 3; andthen of cooling this alloy at a slow rate of 6° C./h, indicated byreference number 4. In this Figure, this cycle is repeated 3 times.

Measurements of the mean grain size made using an image analysistechnique show that repetition of the thermal cycle according to theinvention induces additional growth of the grain of the alloy.Furthermore, measurements of the hardness of the alloy, as a function ofthe number of thermal cycles to which it is subjected, show that thehardness of the alloy reduces with the number of thermal cycles. Table 4below gathers together these measurements.

TABLE 4 Measurements of the grain size and the hardness of the alloy asa function of the number of thermal cycles to which it is subjectedNumber of cycles N 0 1 2 3 4 5 6 Grain size 2.5 8 8.5 9 10 10 10 α (μm)Hardness — 208 195 190 187 185 185 HV (α)

FIG. 4 illustrates the results from this Table. In this Figure, thecurve reference number 10 represents the change in mean grain size ofthe alloy as a function of the number of thermal cycles applied to thisalloy, and the curve, reference number 20 represents the change in thehardness of the alloy as a function of the number of thermal cyclesapplied to this alloy.

These results show that the additional growth of the grain of the alloyis saturated at the end of four thermal cycles according to theinvention (curve 10) and that the hardness of the alloy reduces at eachapplication of a thermal cycle according to the invention (curve 20) andalso stabilizes at the end of four cycles. The grain size achieved is 10μm with six cycles according to the invention and the hardness of thealloy is 185 with this same number of cycles.

EXAMPLE 4

Obtaining a Martensitic ODS Structure According to the Invention

A martensitic ODS structure is obtained from a ferritic ODS structurehaving a grain size of 8 μm, manufactured in the previous example. Theferritic structure is subjected to a thermal treatment according to theinvention comprising an austenitization at a temperature of about 1000°C. for 30 minutes, followed by a fast cooling at a rate greater thanVr(m), in this example at a rate of 700° C./hour.

The method of the invention allows one to obtain an alloy having amatrix with a single phase martensitic structure with lathe lengths thatare much greater than those obtained after austenitization of theas-drawn structure of the EM10+Y₂O₃−ODS alloy. By passing through aferritic ODS structure with large grains one is able to increase thegrain size of the old austenite grain, that is to say, the grain size ofthe high temperature phase that defines the lathe length of themartensitic phase.

Observation with a transmission electron microscope allows one to checkthat the distribution of the Y₂O₃ oxide particles is not modified by themethod of the invention.

On the other hand, the microstructure of the alloy manufactured by themethod of the invention characterized by the presence of equiaxialgrains is isotropic over both parallel and perpendicular sections withrespect to the draw direction of bars manufactured from this alloy.

Therefore the method of the invention allows one to ensure that theanisotropy present in alloys manufactured according to the prior artdisappears, and equivalent mechanical behavior is guaranteed whateverthe direction of the load applied to the alloy. The martensitic alloymanufactured according to the invention has a hardness greater than orequal to 300 after austenitization and tempering.

Hence, depending on the application for the alloy manufactured accordingto the method of the invention, this alloy can be made use of in itsferritic phase or in its tempered martensitic phase.

EXAMPLE 5

Effect of the Grain Size on the Tensile Properties of EM10+Y₂O₃−ODSMartensitic Alloys

The alloys manufactured in Example 4 above have been subjected totensile strength measurements at high temperature. These measurementshave been carried out at 650 to 750° C. on formed bars having differentsizes of the old austenitic grains

Table 5 below gathers together the measurements of this Example. In thisTable, Rp_(0.2%) represents the elastic limit at 0.2% and R_(m)represents the maximum tensile strength. The alloy ODS-MA957 has beenrecrystallised, that is to say, it has been subjected to a specific heattreatment in order to improve its mechanical hot strength. Themeasurement values of this alloy correspond to measurements made along adirection parallel to the forming axis of the tubes or bars. Because ofits anisotropy, these values represent maximum values obtainable for themechanical strength of ODS-MA957.

TABLE 5 Tensile strength measurements at high temperature 650° C. Rp0.2%750° C. Materials (MPa) R_(m) (MPa) Rp0.28 (MPa) R_(m) (MPa)Conventional EM10 190 223 — — without oxide dispersion EM10 + Y₂O₃ − ODS233 285 117 152 grain size ≦ 1 μm EM10 + Y₂O₃ − ODS 305 331 188 205according to the invention ODS-MA957 290 295 180 190 recrystallised

These values show that the martensitic alloy EM10+Y₂O₃−ODS according tothe invention has hot mechanical properties that are better than alloyODS-MA957 of the prior art, the latter having, in addition thedisadvantages already mentioned of anisotropy and embrittlement underneutron irradiation.

Application Example

The method claimed in this patent is directly applicable to themanufacture of tubes that can be used, for example, for the cladding offuel for a classic fast neutron reactor or for future generations ofhybrid reactors, for which a material is demanded that has very highresistance to neutron irradiation within the temperature range 400-700°C. In contrast to the austenitic steels currently used as referencematerials, for example, type 15-15Ti austenitic steels, the martensiticODS alloys claimed in this patent can tolerate the high required dosesof neutron radiation, greater than 200 dpa.

The method of manufacture according to the invention can be applied tothe manufacture of structures that are thicker than cladding. Inparticular the martensitic ODS alloy claimed can be suitable for allnuclear applications that require good mechanical properties underneutron irradiation, for example for the nuts and bolts used inside apressurized water reactor and for a structure that is highly stressed ina fusion reactor.

For all these applications, one may envisage other chemical compositionsbased on the 9 Cr—Mo materials and the variants referred to as “lowactivity” 9 Cr—W type alloys without going outside the scope of theappended Claims.

Finally, outside the nuclear field, the ferritic-martensitic ODS alloysaccording to the invention are suitable for any application thatrequires a high mechanical strength at high temperature, notably inorder to manufacture a component for a thermal power station, and forthe glass, gas and aeronautical industries.

What is claimed is:
 1. A method of manufacturing an alloy with aferritic ODS structure that includes chromium, said method comprising astep of preparation of a martensitic ODS blank that includes chromium byconsolidating a pre-alloyed powder obtained by mechanical alloying, anda step consisting of subjecting said martensitic ODS blank to at leastone thermal cycle, said, at least one thermal cycle comprising anaustenitization of the martensitic ODS blank at a temperature greaterthan or equal to the AC3 point of this alloy in such a way as to obtainan austenite, followed by cooling of this austenite at a slow coolingrate that is less than or equal to the critical cooling rate fortransformation of this austenite into ferrite in such a way as to obtainan alloy with a ferritic ODS structure, said slow critical cooling ratebeing determined from a phase transformation diagram for this austeniteunder continuous cooling.
 2. A method as in to claim 1, in which themartensitic ODS blank that includes chromium includes, in addition, oneor more members of the group comprising Y₂O₃, TiO₂, MgO, Al₂O₃, MgAl₂O₄,HfO₂, ThO₂ and ZrO₂.
 3. A method as in claim 2, in which theaustenitization is carried out at a temperature of from about 1000° C.to about 1250° C. for a period of from about 15 minutes to about 120minutes.
 4. A method as in claim 3, in which the austenitization iscarried out at a temperature of about 1000° C. for a period of about 30minutes.
 5. A method as in to claim 1, in which the martensitic ODSblank that includes chromium includes, in addition, one or more elementschosen from the group comprising Mo, W, Ni, Mn, Si, C, O, N, Y, Ti, Ta,V, Nb, Zr.
 6. A method as in claim 5, wherein an alloy compositioncomprises chromium of from about 7 to about 12% by weight, Mo of fromabout 0.3 to about 1.5% by weight, W of from about 0.5 to about 3% byweight, Ni of up to about 1% by weight, Mn of up to about 1% by weight,Si of up to about 1% by weight, C of from about 0.02 to about 0.2% byweight, O of from about 0.02 to about 0.3% by weight, N of up to about0.15% by weight, Y of up to about 1% by weight and Ti of up to about 1%by weight in the alloy, the remainder being iron.
 7. A method as inclaim 1, in which the martensitic ODS blank that includes chromiumcomprises from about 7 to about 12% by weight of chromium equivalent inthe alloy.
 8. A method as in claim 1, comprising at least two thermalcycles, said thermal cycles being separated at least once, by at leastone forming treatment of the alloy obtained with a ferritic ODSstructure.
 9. A method as in to claim 8, in which the forming treatmentof the alloy with a ferritic ODS structure comprises forming the alloywith a ferritic structure followed by a thermal stress relievingtreatment of the alloy formed at a temperature lower than AC1.
 10. Amethod as in to claim 9, in which the forming of said alloy is a coldforming.
 11. A method as in to claim 9, in which the stress relievingtreatment is a thermal softening treatment at a temperature below about775° C.
 12. A method as in claim 1, in which the slow cooling rate of atleast one thermal cycle is less than or equal to about 280° C. per hour.13. A method as in claim 1, in which the slow cooling rate of at leastone thermal cycle is less than or equal to about 100° C. per hour.
 14. Amethod as in claim 1, in which the slow cooling rate of at least onethermal cycle is less than or equal to about 20° C. per hour.
 15. Amethod as in claim 1, in which the preparation of the martensitic ODSblank that includes chromium is carried out by mechanical alloying of apre-alloyed powder obtained by atomization under argon of an ingot, thathas a composition corresponding to that of the manufactured alloy andadding oxides.
 16. A method of manufacturing a nuclear fuel claddingcomprising an alloy with a ferritic ODS structure that includeschromium, said method comprising a method of manufacturing said alloy asin claim
 1. 17. A method of manufacturing a martensitic ODS alloy thatincludes chromium, this method comprising a method of manufacturing analloy with a ferritic ODS structure according to any one of claims 1 to12, followed by a martensitic transformation step and a tempering of themanufactured martensitic ODS alloy, said martensitic transformation stepcomprising an austenitization of the alloy with a ferritic ODS structureat a temperature greater than or equal to the AC3 point of this alloy insuch a way as to obtain an austenite, followed by cooling of thisaustenite at a fast rate of cooling greater than or equal to thecritical cooling rate for transformation of austenite into martensite,said fast rate of cooling being determined from a phase transformationdiagram for this alloy under continuous cooling.
 18. A method as in toclaim 17, in which the fast rate of cooling is greater than or equal toabout 700° C. per hour.
 19. A method as in claim 17, in which thetempering is carried out at a temperature of about 750° C. for about 1hour.